Pressure swing adsorption process and system for gas separation

ABSTRACT

A pressure swing adsorption process and system including at least two adsorption beds and a segregated storage adsorption bed which is isolated from direct communication with the feed gas stream. During the process the pressures in the adsorption beds are equalized from the feed ends thereof at the end of adsorption in one of the beds and after pressurization of the other bed. The segregated storage adsorption bed is pressure equalized with a depressurizing adsorption bed and then after purging of the bed the segregated storage adsorption bed is equalized with that adsorption bed during repressurizing thereof. A pair of flow control valves are connected in a gas flow path connected to the outlets of the adsorption beds, each valve being located adjacent a corresponding one of the beds and allowing unrestricted flow away from the corresponding bed and controlled flow toward that bed. A reservoir connected to the system output conduit stores product gas for use during a system malfunction or for augmenting system function.

This is a continuation of application Ser. No. 874,167, filed Feb. 1,1978 and now U.S. Pat. No. 4,194,890, which was a continuation ofapplication Ser. No. 745,285 filed on Nov. 26, 1976 and now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to the art of separation of gas mixtures, andmore particularly to a new and improved process and system forseparating gas mixtures by pressure swing adsorption.

One area of use of the present invention is in separating air to providea product stream of high purity oxygen, although the principles of thepresent invention can be variously applied. In basic pressure swingadsorption processes and systems for separating air, adsorption iscarried out at a high pressure and desorption is carried out at a lowpressure. Compressed air is introduced into a fixed bed of adsorbentmaterial and nitrogen is then preferentially adsorbed to produce oxygenrich gas product. When the adsorption bed is about saturated, the bedpressure is reduced to desorb nitrogen from the adsorbent material andregenerate the adsorption capacity. To increase the efficiency ofregeneration, a purge by some of the product or an intermediate processstream often is used. To facilitate continuous operation, two or moreadsorption beds are employed so that while one bed performs adsorptionthe other bed undergoes regeneration.

In the design and operation of pressure swing adsorption processes andsystems, it would be highly desirable to provide maximum utilization ofadsorbent material in the adsorption beds, reduction in energyrequirements for operation of the system, a substantially constantdegree of product purity, and reduction in adsorbent materialrequirements while maintaining a high degree of product purity, alongwith improved efficiency and reliability.

SUMMARY OF THE INVENTION

It is, therefore, a primary object of this invention to provide a newand improved process and system for separation and fractionation of gasmixtures by pressure swing adsorption.

It is a further object of this invention to provide such a process andsystem characterized by maximum utilization of adsorbent material in theadsorption beds.

It is a further object of this invention to provide such a process andsystem having reduced energy requirements for operation.

It is a further object of this invention to provide such a process andsystem which is balanced and yields a substantially constant degree ofproduct purity.

It is a further object of this invention to provide such a process andsystem which has reduced adsorbent material requirements along with ahigh degree of product purity.

It is a further object of this invention to provide such a process andsystem which maintains a reserve supply of product for use during asystem malfunction or in augmenting system functions.

It is a further object of this invention to provide such a process andsystem which is reliable, efficient and economical.

The present invention provides a pressure swing process and system forfractionating at least one component from a gaseous mixture by selectiveadsorption in each of at least two adsorption beds the gas inlets ofwhich are selectively connected to a feed gas stream and the gas outletsof which are coupled by output conduit means to a gas product outlet. Asegregated storage adsorption bed is connected at one end selectively tothe outlets of the adsorption beds and is isolated from directcommunication with the feed gas stream. The pressures in the adsorptionbeds are equalized from the feed ends thereof at the end of adsorptionin one of the beds and after pressurization of the other bed. The oneadsorption bed and the segregated storage adsorption bed are equalizedin pressure from the outlet end of the one bed while product iswithdrawn from the outlet of the other bed. The one adsorption bed thenis depressurized and purged while product is withdrawn from the otherbed. The segregated storage adsorption bed and the one adsorption bedare equalized in pressure through the outlet end of the one bed. Theforegoing steps are repeated consecutively reversing the functions ofthe two beds. A pair of flow control valves are connected in a gas flowpath connected to the outlets of the two adsorption beds, each valvebeing located adjacent a corresponding one of the beds and allowingunrestricted flow away from the corresponding bed and controlled flowtoward that bed. A reservoir connected to the output conduit meansstores product gas which can be supplied to the product outlet during amalfunction or to the adsorption beds for augmenting system functions.

The foregoing and additional advantages and characterizing features ofthe present invention will become clearly apparent from the ensuingdetailed description wherein:

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a schematic diagram of a pressure swing adsorption systemaccording to the present invention;

FIG. 2 is a cycle sequence chart illustrating the pressure swingadsorption process of the present invention;

FIG. 3 is a schematic diagram of a pressure swing adsorption system withparts removed according to another embodiment of the present invention;and

FIG. 4 is a schematic block diagram of a control arrangement accordingto the present invention for a pressure swing adsorption system.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Referring now to FIG. 1, there is shown a system according to thepresent invention for fractionating at least one component from agaseous mixture by pressure swing adsorption. The gaseous mixture issupplied to the system by a feed gas stream which flows along an inputconduit 10 and is moved therealong by means of a pump or compressor 12.Although the present system and process is specifically described andillustrated in relation to the application of pressure swing adsorptionto the fractionation of air to produce an oxygen rich stream, thepresent invention is broadly applicable to the separation of organicand/or inorganic gas mixtures.

The system includes a first adsorption bed 16, also designated A, havinga gas inlet 18 and a gas outlet 20. The system further includes at leastone additional adsorption bed 24, also designated B, having a gas inlet26 and a gas outlet 28. Adsorption beds A and B are the type comprisinga vessel containing adsorbent material and are well known to thoseskilled in the art. A preferred vessel construction includes an outerpressure cell with an inner annulus, and one skilled in the art canprovide suitable pressure vessels, piping or tubing, connectors, valvesand auxiliary devices and elements. Likewise, adsorbent materials arewell-known in the art, and one skilled in the art may select anadsorbent material(s) which is commercially recommended for theseparation or fractionation of the particular gas to be purified.Examples of typical adsorbent materials for use in adsorption bedsinclude natural or synthetic zeolites, silica gel, alumina and the like.Generally, the adsorbent beds in a system contain the same adsorbentmaterial, however, each bed may contain a different type of adsorbentmaterial or different mixtures of adsorbent material as desired. Theparticular adsorbent material or mixtures used are not critical in thepractice of this invention as long as the material separates orfractionates the desired gas components.

The system of the present invention further comprises a segregatedstorage adsorption bed 32, also designated C, and in the system shown inFIG. 1 gas is introduced to and withdrawn from the segregated storageadsorption bed C at the same end which is provided with a conduit 34.The segregated storage adsorption bed C likewise is a vessel containingadsorbent material, but bed C does not communicate with the feed gasstream from conduit 10. In the system shown, adsorption bed C isapproximately the same size as the adsorption beds A and B and maycontain the same type of adsorbent material, but the segregated storageadsorption bed C can be smaller in size, include different adsorbentmaterial, and be operated at a different capacity as compared to theadsorption beds A and B.

The gas inlet 18 of adsorption bed A is connected to conduit 10containing the feed gas stream by suitable conduit means including anautomatic valve 40A and, similarly, the gas inlet 26 of adsorption bed Bis connected to the feed gas stream in conduit 10 by suitable conduitmeans including an automatic valve 40B. The system further includes awaste gas outlet 44 which can be open to the atmosphere or which can bein fluid communication with a waste gas stream. The gas inlets 18 and 26of adsorption beds A and B, respectively, also are connected to thewaste gas outlet 44 by suitable corresponding conduit means includingautomatic valves 46A and 46B, respectively. The automatic valves 40 and46 and those additional automatic valves to be described can be of thesolenoid-operated type, but in any event are of the type which areoperated to be either fully open or fully closed.

The system of the present invention further comprises means such assuitable conduits or piping defining a gas flow path connected at oneend to gas outlet 20 of adsorption bed A and connected at the oppositeend to gas outlet 28 of adsorption bed B. A first flow control valve 50Ais in the gas flow path between gas outlet 20 of adsorption bed A andadsorption bed B. Valve 50B allows unrestricted gas flow in a directionfrom the outlet 20 of bed A through the valve toward adsorption bed B,and the valve provides controlled flow therethrough in a direction togas outlet 20 of adsorption bed A. The controlled flow preferably isprovided by manual adjustment. A second flow control valve 50B is in thegas flow path between gas outlet 28 of adsorption bed B and theadsorption bed A. Valve 50B allows unrestricted gas flow therethrough ina direction from gas outlet 28 of adsorption bed B toward adsorption bedA, and it provides controlled flow therethrough in a direction to gasoutlet 28 of adsorption bed B. The controlled flow preferably isprovided by manual adjustment. Valves 50A, 50B preferably are identicaland can be of the type known commercially as Parker-Hannifin flowcontrol valves. An isolation valve in the form of an automatic valve 54is provided in the gas flow path between gas outlets 20 and 28 of theadsorption beds, and in the system shown valve 54 is connected betweengas outlet 20 of adsorption bed A and the flow control valve 50A.

The system of the present invention includes a second gas flow pathprovided by suitable conduits or piping which joins the gas outlets 20and 28 of the adsorption beds A and B, respectively. A first automaticvalve 60A is connected in the path adjacent outlet 20 of bed A, and asecond automatic valve 60B is connected in the path adjacent outlet 28of adsorption bed B. The segregated storage adsorption bed C isconnected through an automatic valve 82 to a point in the gas flow pathbetween the automatic valves 60A and 60B.

The system of the present invention further comprises a product outletdesignated 66 and output conduit means for coupling the gas outlets ofthe adsorption beds to the product outlet 66. In the system shown theoutput conduit means is connected to the first flow path at a pointbetween the flow control valves 50A and 50B and includes a first section70 including an automatic valve 72 and a second section 74 including theseries combination of a pressure regulator 76, a throttle valve 78 and aflow meter 80. The flow rate of product to the outlet 66 is controlledby valve 78 which preferably is a manually adjustable needle-type valve,and the flow rate is indicated visually by the meter 80.

The system of the present invention further comprises a reservoir 84which functions primarily to store product gas received through aconduit 86 and serve as a reverse supply of product for use in the eventof a system malfunction. A first reservoir conduit means is connected atone end of the system output conduit means and at the other end to thereservoir 84 through conduit 86 and includes flow control means in theform of check valve 90 which allows gas flow only in one direction fromthe system output conduit means to the reservoir 84. Another valve 92 inthe form of a throttle valve which preferably is manually adjustable isconnected in the conduit and preferably between check valve 90 andreservoir 84. Valve 92 can be used to control the rate of flow of gasproduct into reservoir 84. A second reservoir conduit means is connectedat one end to reservoir 84 through conduit 86 and at the other end tothe system conduit means and includes valve means 96 for controlling theflow of product gas from reservoir 84 to the output conduit means. Acontrol 100 is connected by lines 102 and 103 to valves 72 and 96,respectively, and functions to open the normally closed valve 96 inresponse to closing of valve 72. A pressure indicator meter 104 can beconnected to the output of reservoir 84 for the purpose of indicatingthe pressure of gas product remaining therein.

In general, the present invention is illustrated in terms of a processand system utilizing a first adsorption bed, a second adsorption bed anda segregated storage adsorption bed. However, the process and system canemploy more than one first adsorption bed, more than one secondadsorption bed and more than one segregated storage adsorption bed. Theadsorption beds communicate with the feed gas stream which supplies thegaseous mixture, and the segregated storage adsorption bed neverdirectly communicates with or is directly exposed to the feed gasstream.

Although the process and system of the present invention are describedwith particular reference to separation or fractionation of air toprovide a high purity product oxygen by removal of nitrogen, essentiallyany gas mixture may be separated by the process and system of thepresent invention by the proper selection of time for each cycle andstep and by the selection of a proper adsorbent material, adsorbentmaterials or mixtures of adsorbent materials.

As used herein, depressurizing or depressurization refers to thereduction of pressure in the vessel and associated piping of anadsorption bed and the level to which pressure is reduced can beselected by those skilled in the art depending upon operating conditionsand the nature of the gas mixture being fractionated. Desorption andpurging pressures are selected in a similar manner. Pressurizing orpressurization refers to the increase of pressure in the vessel andassociated piping of an adsorption bed. The process and system of thepresent invention have the capability of product gas delivery in a lowpressure range down to about 2 p.s.i.g. and in a high pressure range upto about 4 p.s.i.g. The present invention is not limited to particularpressures of the product gas or any other pressures, and one skilled inthe art can manipulate and adjust pressures throughout the system toprovide the desired delivery or product gas pressure. For example, whenair is fractionated to deliver high purity oxygen gas product, adelivery pressure of around 3 p.s.i.g. is employed for medical uses andbreathing devices whereas a higher delivery pressure of up to about 40p.s.i.g. is ideally suited for commercial uses such as in metal cuttingor welding equipment.

FIG. 2 illustrates a process timing sequence according to the presentinvention for use with the system of FIG. 1. In FIG. 2 preferred timesin seconds are indicated for each step, and preferred pressures in eachadsorption bed for each step are shown parenthetically and given inpounds per square inch gage. The particular operation carried out ineach adsorption bed during each step is shown in FIG. 2, most of whichare abbreviated for convenience in illustration. Thus "FEE" refers tofeed end equalization and will be explained in further detail presently,"ISOL" refers to isolation of a particular adsorption. "EQ" refers topressure equalization of two adsorption beds and will be explained infurther detail presently, "REP" refers to repressurization orrepressurizing to increase the pressure in an adsorption bed, and"PURGE" refers to introduction of purge gas or purging.

Referring now in detail to FIG. 2, prior to step No. 1 the gaseousmixture i.e. ordinary air, has been flowing from the feed gas stream inconduit 10 and through valve 40A which is open into and throughadsorption bed A wherein nitrogen is adsorbed. High purity oxygen gasleaves bed A through outlet 20 and flows through the opened valve 54 andflow control valve 50A and then flows along conduit section 70 throughthe opened valve 72, along conduit section 74 and through the seriescombination of pressure regulator 76, needle valve 78 and flow meter 80to the product outlet 66 for use. Just prior to the beginning of stepNo. 1, adsorption bed A is about saturated and nearing the end of theadsorption operation therein. Also just prior to the beginning of stepNo. 1, adsorption bed A is at a higher pressure than the adsorption bedsB and C.

At the beginning of step No. 1, valve 40B is opened, and valve 40A iskept open as well as valve 72. As indicated in FIG. 2, at the beginningof step No. 1, typical pressures in beds A, B and C are 30, 7 and 7,respectively.

During this step, gas flows from the bottom or feed end of adsorber A ina reverse direction through valve 40A whereupon it mixes with theincoming feed air stream from conduit 10 and flows through valve 40Binto the bottom or feed end of adsorber B. Adsorption bed A is very nearthe end of the adsorption step therein, and the composition of this gaswithdrawn from inlet 18 thereof is not appreciably different from thecomposition of air. As a result, during this step, adsorption bed A isdepressurized countercurrently to feed flow, and adsorption bed A ispressure equalized with adsorption bed B causing the pressure in bed Bto rise. Also in this step, adsorption bed A continues to supply oxygengas product, but this is terminated by the end of the step. Step no. 1preferably has a duration of about 7 seconds. Throughout this step andall other steps there is continuous air flow into the system andcontinuous product flow out. Cocurrent to feed flow is in a directionfrom the inlet to the outlet of the adsorption bed and countercurrent tofeed flow is in a direction from the outlet to the inlet of theadsorption bed.

The process of step no. 1 may be described as continuing to dischargeproduct gas from the outlet of the first bed while simultaneouslyequalizing the pressures of the first and second adsorption beds fromthe feed ends thereof by withdrawing gas from the feed inlet of thefirst adsorption bed at the end of the adsorption operation therein in adirection countercurrent to feed flow and introducing the withdrawn gasalong with the gaseous mixture from feed gas stream to the feed inlet ofthe second adsorption bed in a direction cocurrent with feed flow andafter pressurization thereof.

As shown in FIG. 2, at the transition between the end of step no. 1 andbeginning of step no. 2, the pressures in beds A and B are equalized at20 p.s.i.g. and the pressure in the segregated storage adsorption bed Chas remained at 7 p.s.i.g. At the beginning of step no. 2, valve 40Bremains open, valve 40A closes, and valve 60A opens. No product gas isobtained from adsorption bed A. During this step, feed air continues toflow into the feed inlet 26 of adsorber B, and oxygen rich gas is takenas product from the outlet 28 of adsorber B and flows through flowcontrol valve 50B into conduit section 70 and through the remainingsystem components as previously described to product outlet 66. At thesame time, low purity gas flows from the outlet 20 of adsorber A throughvalve 60A and valve 62 into the segregated storage adsorption bed C. Asa result, during this step adsorption bed A is pressure equalized withthe segregated storage adsorption bed C. The automatic valve 62 canremain open during all steps or it can be opened and closed whennecessary. Step no. 2 preferably has a duration of about 7 seconds.

The process of step no. 2 may be described as simultaneously terminatingthe pressure equalization of step 1, adsorbing the gaseous mixture fromthe feed gas stream in the second adsorption bed, releasing product gasfrom the outlet of the second adsorption bed, and equalizing thepressures of the first adsorption bed and the segregated storageadsorption bed by withdrawing low purity gas from the outlet of thefirst adsorption bed in a direction cocurrent with feed flow andintroducing the low purity gas into the segregated storage adsorptionbed.

As shown in FIG. 2, at the transition between the end of step no. 2 andthe beginning of step no. 3, the pressures in adsorption beds A and Care equalized at 14 p.s.i.g. and the pressure in adsorption bed B hasrisen to 28 p.s.i.g. At the beginning of step no. 3, valve 40B remainsopen, valve 60A closes and valve 46A opens. During this step feed aircontinues to enter bed B, and product quality oxygen rich gas continuesto be taken as product from the outlet of bed B and is available atproduct outlet 66. Also during this step, adsorption bed A isdepressurized to the atmosphere through valve 46A and waste outlet 44 ina direction countercurrent to feed flow. As a result, nitrogen richwaste gas is rejected to the atmosphere, and the pressure in adsorber Adrops from 14 p.s.i.g. to 0 p.s.i.g. Concurrently with the foregoingdepressurization, a portion of the oxygen gas product flowing fromadsorber B through flow control valve 50B flows through valve 50A andvalve 54 into adsorber A. The product quality oxygen gas flows throughbed A and out through valve 46A and waste outlet 44 in a directionopposite to that of air separation. This oxygen purge flowingcountercurrent to feed flow displaces nitrogen from the adsorbentmaterial in bed A, and nitrogen rich stream leaves the system throughvalve 46A and outlet 44 to the atmosphere. As a result, product qualityoxygen gas is taken from the adsorbing bed B to purge the nitrogenloaded bed A in a reverse direction to reject unwanted impurity to theatmosphere. Step no. 3 preferably has a duration of about 39 seconds.

The process of step no. 3 may be described as simultaneously terminatingthe pressure equalization of step 2, continuing adsorption of thegaseous mixture from feed gas stream in the second adsorption bed,releasing product gas from the outlet of the second adsorption bed, anddepressurizing the first adsorption bed in a direction countercurrent tofeed flow and purging the first adsorption bed by diverting some productgas from the outlet of the second adsorption bed into the firstadsorption bed in a direction countercurrent to feed flow.

As shown in FIG. 2, at the transition between the end of step no. 3 andthe beginning of step no. 4, the pressure in bed A is at 0 p.s.i.g., thepressure in segregated storage adsorption bed C has remained at 14p.s.i.g., and the pressure in bed B has risen to 30 p.s.i.g. At thebeginning of step no. 4, valve 40B remains open, valve 46A closes, andvalve 60A opens. Valve 62 if not already open is opened at the beginningof this step. During this step feed air continues to enter bed B, andproduct quality oxygen gas continues to be taken as product from theoutlet of bed B and is available at product outlet 66. At the same time,gas flows from the segregated storage tank C through valves 62 and 60Ainto adsorber A through the outlet 20 thereof. This gas withdrawn fromadsorption bed C during step 4 is a version of the gas supplied to bed Cduring step 2 which gas has been influenced by travel in bed C.

As a result, during this step the segregated storage adsorption bed C ispressure equalized with the adsorption bed A. At least during theinitial portion of step 4, there is some additional flow of gas from bedB through valves 50B, 50A and 54. At least during the initial portion ofstep no. 4, there is some additional flow of purge gas from bed Bthrough valve 50B, 50A and 54. Step no. 4 preferably has a duration ofabout 7 seconds.

The process of step no. 4 may be described a simultaneously terminatingthe depressurizing and purging of the first adsorption bed, continuingadsorption of the gaseous mixture from the feed gas stream in the secondadsorption bed, releasing product gas from the outlet of the secondadsorption bed and equalizing the pressures of the segregated storageadsorption bed and the first adsorption bed by withdrawing gas from thesegregated storage adsorption bed and introducing the withdrawn gas intothe first adsorption bed in a direction countercurrent to feed flow.

The foregoing process steps are repeated consecutively beginning withpressure equalization of the adsorption beds from the feed ends thereofreversing the functions of the adsorption beds A and B. In particular,as shown in FIG. 2, at the transition between the end of step no. 4 andthe beginning of step no. 5, the pressures in beds A and C are equalizedat 7 p.s.i.g. and the pressure in bed B has remained at 30 p.s.i.g. Atthe beginning of step no. 5, valve 40B remains open, valve 60A closesand valve 40A opens. During this step, gas flows from the bottom or feedend of adsorber B, which is near the end of its adsorption operation, ina reverse direction through valve 40B whereupon it mixes with theincoming feed air stream from conduit 10 and the resulting mixture flowsthrough valve 40A into the bottom or feed end of adsorber A. As aresult, adsorption bed B is pressure equalized with adsorption bed A,and bed A begins to adsorb the feed gas mixture. This feed endequalization is similar to that which occurred during step no. 1 but inthis step the roles of the beds A and B are interchanged. Also duringthis step, product quality oxygen rich gas continues to be taken asproduct from bed B and is available at product outlet 66. This stepbegins the second half of the process cycle wherein steps 5-8 aresimilar to 1-4 with the roles of beds A and B interchanged and with thevalve sequence being the same with the A and B designationsinterchanged.

For example the process of step no. 6 (the same as step 2 with bedsreversed) may be described as simultaneously terminating the pressureequalization of step no. 5, repressurizing the first adsorption bedwhile withdrawing product gas therefrom, and equalizing pressures in thesecond adsorption bed and the segregated storage adsorption zone.

Equalizing the pressures of the adsorption beds A and B at the feed endsthereof according to the present invention, as illustrated in step no.1, advantageously reduces energy requirements and increases oxygenrecovery. When an adsorption bed at the end of the adsorption steptherein is depressurized countercurrently to feed flow, i.e. as bed Afrom 30 p.s.i.g. to 20 p.s.i.g. in step no. 1, the composition of thegas obtained from the bed inlet is not greatly different from air.Therefore this gas can be introduced into the feed end of arepressurizing adsorber, i.e. adsorption bed B in step no. 1, withoutany appreciable loss in system performance compared to repressurizingwith air from the system compressor 12. Feed end equalization accordingto the present invention thus greatly reduces the feed air requirementand increases oxygen recovery, i.e. decreases the size of compressor 12required to produce a given amount of oxygen. Feed end equalizationrecovers energy, increases system efficiency and can be used for bothlow and high product delivery pressures. The foregoing advantages ofcourse apply to both of the feed end equalizations which occur during asingle cycle as illustrated in step nos. 1 and 5.

The feed end equalization according to the present invention requiresless adsorbent material in a given bed as compared to product endequalization for the following reasons. In product or outlet endequalization, the bed at the higher pressure depressurizes in adirection cocurrent to feed flow during the pressure equalization step.This causes the mass transfer zone to advance toward the product end ofthe bed as the pressure decreases. In order to contain the mass transferzone during this step to maintain product purity, a larger bed, i.e.more adsorbent material, is required. In feed end equalization accordingto the present invention, on the other hand, the bed at the higherpressure depressurizes in a direction countercurrent to feed flow duringthe equalization step. In this step the mass transfer zone does notadvance due to the direction of the gas flow. The countercurrentdepressurization also is beneficial for the subsequent purge stepbecause nitrogen starts to flow toward the feed end of the bed duringthis step. The combination of no advancing of the mass transfer zone andcountercurrent depressurization reduces the amount of adsorbent materialrequired.

Bed size factor is a quantity used to compare the amount of adsorbentmaterial required from one system or cycle to another. At a given bedsize factor, it has been determined that using feed end equalizationaccording to the present invention produces oxygen at a higher purity ascompared to using product end equalization.

The combination of equalizing pressures of an adsorption bed and thesegregated storage adsorption bed when the adsorption bed is at the endof the adsorption operation therein and prior to purging of the bed asillustrated in step no. 2 and thereafter equalizing pressures betweenthese same two components after purging of the adsorption bed when it isat a relatively low pressure as illustrated in step no. 4 maximizes theutilization of the adsorption bed while at the same time maximizingpurity of the product. In particular, during step no. 2 as thedepressurizing adsorber A equalizes cocurrently to feed flow intosegregated storage adsorption bed C, part of the nitrogen contained inthe mass transfer zone of bed A will be transferred into the bed C. Thisallows for maximum and continual utilization of adsorption bed A, i.e.the mass transfer zone can be moved along bed A from inlet to outlet asfar as possible. At the beginning of the flow from bed A to bed C thegas is rich in oxygen but as flow continues the gas becomes more likeair. In addition, the segregated storage adsorption bed recovers somepotential energy from the depressurizing adsorber and this, in turn,reduces system blowdown pressure and increases recovery and efficiency.Providing the segregated storage adsorption bed C in effect provides amixing volume to smooth out any fluctuations in product purity whichotherwise might occur when the front of the mass transfer zone breaksout of the output end of an adsorption bed. The foregoing advantagesresult when the system is operating at equilibrium conditions and atflow conditions for which the system is optimally designed. For example,when the system is used to supply oxygen for medical use, designconditions occur at a flow rate of about 3.0 liters per minute.

During step no. 4 as the segregated storage adsorption bed C pressureequalizes countercurrently to feed flow into adsorber A, the gasreturned to adsorber A is distributed or dispersed therethrough in amanner which does not adversely affect product purity. The gas is notreturned to adsorber A in a lump quantity concentrated in the outputregion of bed A but instead is spaced, equalized or dispersed throughand along the bed A. The foregoing is believed to result from the factthat gas return to adsorber A occurs when the latter is at a relativelylow pressure, i.e. 0 p.s.i.g. after purging of adsorber A, which lowpressure allows the gas to disperse through the bed. It is believed thatlow or zero pressure in bed A allows the incoming gas to move along thebed in a manner such that a large amount of nitrogen is not taken up bythe adsorbent material adjacent the outlet end of the bed. At thebeginning of gas flow from bed C to bed A, the gas is rich in nitrogenbut as the flow continues it becomes more rich in oxygen. The foregoingadvantages are of course equally associated with the relationshipbetween adsorption bed B and segregated storage adsorption bed C duringstep nos. 6 and 8.

Providing the flow control valves 50A and 50B allows the system to bebalanced by providing individual control or adjustment of the purge gasflow to each of the adsorption beds A and B. Providing an adjustableflow control valve associated with each bed permits compensating fordifferences in the beds and piping by simple manual adjustment of valves50A, 50B. An unbalanced system is characterized by the front of the masstransfer zone breaking through the output end of one bed sooner than inthe other bed. In order to maintain purity, this would limit systemoperation to that of the bed which is first to experience nitrogenbreakthrough thereby causing the other adsorber to be underutilized withthe result that the entire system produces less oxygen at a givenpurity. System balance and optimization are achieved by theindependently adjustable flow control valves 50A, 50B. Advantageously,product gas also travels through these same valves toward the systemproduct outlet 66. Alternatively, flow control valves 50A and 50B couldbe replaced by two needle valves for independently controlling purgeflow and then the combination of two check valves would be converted inparallel with the needle valves and oriented to transmit product gasfrom the bed outlets to the system product outlet 66.

The automatic valve 54 in the path containing valves 50A, 50B is a shutdown isolation valve which serves to isolate beds A and B when thesystem is shut down to maintain the respective pressures in the beds andprevent pressure equalization. When the system is shut down, all theother automatic valves close also. Then when the system is placed inoperation, less time is required to reach desired operating conditionsby virtue of the beds A and B having been maintained at the respectivepressures prior to shut down.

Table I presents data illustrating the effect of the segregated storagetank or segregated storage adsorption bed C on system performance. Thedata presented in Table I is for oxygen product at a purity of 90% andthe oxygen recovery in percent is presented for both low pressure andhigh pressure delivery conditions. The abbreviations S.S.T. forsegregated storage tank and F.E.E. for feed end equalization are used.

                  TABLE I                                                         ______________________________________                                                         Low      High                                                                 Pressure Pressure                                                             Delivery Delivery                                            ______________________________________                                        S.S.T. Absent      21%        21%                                             S.S.T. Present                                                                But Empty          25%        23%                                             S.S.T. Half Full                                                              Of Adsorbent Material                                                                            35%        31%                                             S.S.T. Full And                                                               With F.E.E.        49%        48%                                             ______________________________________                                    

FIG. 3 shows a system according to another embodiment of the presentinvention wherein gas product can be withdrawn from the other end of thesegregated storage adsorption bed. In the system shown in FIG. 3,components identical to those of FIG. 1 are provided with the samereference numerals but with a prime designation. In addition, the systemof FIG. 3 would also include adsorption beds identical to thosedesignated A and B in the system of FIG. 1, along with similarconnections of the feed gas stream to the gas inlets of the beds,connections of the gas inlets to the waste outlet, and connections ofthe gas outlets of the beds to the gas flow path containing the flowcontrol valves 50A' and 50B'. Thus, the arrowheads at opposite ends ofthe path shown in FIG. 3 containing automatic valves 60A', 60B' and thepath containing flow control valves 50A' and 50B' indicate connection tothe gas outlets of the corresponding adsorption beds A and B. Similarly,the output of regulator 76' is connected through a throttle valve andflow indicator to a product outlet as indicated by the arrowhead in theportion 74' of the gas flow path.

The opposite end of the segregated storage adsorption bed C' isconnected by a conduit 108 which contains an automatic valve 110 to theoutput conduit means, in particular to portion 74' thereof and upstreamfrom regulator 76'. Upon opening of valve 110, product quality gas canbe withdrawn from the segregated storage adsorption bed C' andintroduced to the output conduit means. Withdrawing product gas from thesegregated storage adsorption bed can be advantageous in situationswhere low pressure rather than high pressure product delivery is needed.In addition, when product is delivered from the segregated storageadsorption bed C', the bed serves also as a product surge tank enablingproduct to be withdrawn from the system at a high flow rate for a shortperiod of time before the mass transfer zone breaks through that end ofthe bed. On the other hand, recovery from a breakthrough condition canbe relatively slow. Another advantage of withdrawing product gas fromthe segregated storage adsorption bed C' is that it provides arelatively higher rate of recovery of product. This is becausewithdrawal of product from bed C' reduces the pressure therein so thatwhen the pressure equalizes with either of the adsorption beds thatadsorption bed, in time, will be at a lower pressure. The lowerpressure, in turn, imposes a lower blowdown requirement for that bedwith a result that less gas is released to the atmosphere. Thisreduction in the waste losses, in time, results in a higher percentageof product recovery. Another advantage associated with the segregatedstorage adsorption bed involves feed end equalization which lowers thefront of the mass transfer zone in each of the other two beds so thatwhen the beds are equalized from the tops with the segregated storageadsorption bed there is less nitrogen to be taken up by the segregatedstorage adsorption bed.

As shown in FIG. 3, the system can also include a third reservoirconduit designated 114 connected at one end to the reservoir 84' andcoupled at the other end to the adsorption beds. In the presentillustration, the other end of conduit 114 is connected to the flow pathcontaining the automatic valves 60A' and 60B' and is connected betweenthese valves. Conduit 114 contains an automatic valve 116. Upon openingof valve 116, product gas from reservoir 84' flows to the adsorptionbeds and it can be used for operations such as purging andrepressurization.

The primary role of the reservoir in the system of the present inventionis a reserve supply of product gas in the event of equipment malfunctionor power failure. This is of particular importance when the system ofthe present invention supplies oxygen for medical use. Under normaloperating conditions the reservoir is at a pressure of 28-29 p.s.i.g.,and product oxygen flows through valve 72 and regulator 76 to productoutlet 66. If electrical power is interrupted, valve 72 closes and thisis sensed by control 100 which opens valve 96. Oxygen flow continuesfrom the reservoir through valve 96 to the output conduit to outlet 66until the supply in the reservoir is depleted. An alarm can be soundedto indicate the power interruption.

The reservoir also can be used to supply part or all of the purge oxygenrequired for an adsorber during its purge step. This is accomplished byopening valve 116 at the appropriate time. The reservoir also can beused as another surge tank. Pressure equalizations to and from theadsorbers can be accomplished through the correct sequencing of valves116 and 62.

The primary purpose of the reservoir is a reserve oxygen supply in theevent of a malfunction. The length of time the reserve oxygen lastsdepends on the pressure in the reservoir at the time of the malfunction.If the reservoir is used only as a back-up oxygen supply, the reservoirpressure will be at its maximum at all times. If the reservoir is usedto supply supplemental purge and or repressurization gas, the pressurein the reservoir will vary as will the reserve supply of oxygen. Thereservoir can comprise an adsorption bed but it also can comprise andordinary tank of larger size.

FIG. 4 shows an arrangement for controlling the system and process ofthe present invention. The output conduit means can be connected to atank or similar storage receptacle or vessel 120 and gas product can bewithdrawn therefrom through a conduit or path 122 for use. Thesequencing and timing of the system including the control of theautomatic valves is performed by a system control designated 124, andcontrol signals or commands generated by the control 124 are transmittedby lines collectively designated 126 to the valves and other appropriatecomponents of the system. Persons skilled in the art are readilyfamiliar with such controls so that a detailed description thereof isbelieved to be unnecessary. Generally, the control 124 is responsive tothe pressure of product gas within the storage element 120, and to thisend a pressure sensor 130 is operatively connected to the storageelement 120 by the connection designated 132. In accordance with thepresent invention, the output from sensor 130 is connected by a line 134to an additional control means 136 which, in turn, is connected incontrolling relation to the system control 124 by the connectiondesignated 138. In accordance with the present invention, it has beendetermined that once operation of the process and system has begun thereis an optimum time at which to terminate operation, both in terms of aminimum number of cycles to be completed and a point within a cycle toterminate operation. The additional control functions to cause thesystem control 124 to maintain operation of the system, once begun, fora predetermined number of cycles. It has been determined that in asystem of the present invention for producing oxygen from feed air thata total of two complete cycles provides desirable results. One completecycle includes step nos. 1-8 described in FIG. 2. Furthermore, it hasbeen determined that there is an optimum point within a cycle at whichoperation of the system and process should be terminated. This is whenthe pressures are equal in the two adsorption beds A and B which is atthe beginning of step nos. 2 and 6 described in FIG. 2. Thus, theadditional control 136 also functions to stop the system only after twocomplete cycles have been completed and only at an optimum point withinthe next cycle when the pressures are equal in the two adsorption beds Aand B. The additional control can be of the cam type or step switchtype, for example, and persons skilled in the art are readily familiarwith the construction and operation of these and other types which canbe used for additional control 124 so that a detailed descriptionthereof is believed to be unnecessary. Thus, the system control means124 is responsive to gas pressure in storage means 120 signalled bysensing means 130 for stopping operation of the process and systemnormally when gas pressure in storage means 120 reaches a predeterminedmagnitude. The additional control means 136 overrides the system controlmeans to terminate operation of the process and system only at apredetermined time.

It is therefore apparent that the present invention accomplishes itsintended objects. While several embodiments of the present inventionhave been described in detail, this is for the purpose of illustration,not limitation.

We claim:
 1. In a pressure swing process for fractionating at least onecomponent from a gaseous mixture by selective adsorption in each of atleast two adsorption zones by sequentially passing the gaseous mixturefrom a feed stream through a first adsorption zone until the zone isabout saturated while simultaneously purging and then pressurizing asecond adsorption zone and then passing the gaseous mixture from thefeed stream through the second adsorption zone until the zone is aboutsaturated while simultaneously purging and then pressurizing the firstadsorption zone, the improvement comprising:(a) withdrawing low puritygas from one of the adsorption zones in a direction cocurrent with feedflow when the zone is at the end of the adsorption operation therein andprior to purging of the zone until the mass transfer zone moves out ofsaid adsorption zone; (b) introducing the withdrawn low purity gas andsaid mass transfer zone to one end of a segregated storage adsorptionzone; and (c) withdrawing gas from said one end of the segregatedstorage adsorption zone and passing said withdrawn gas and said masstransfer zone into said one adsorption zone in a directioncountercurrent to feed flow after purging of the zone and when the zoneis at a relatively low pressure.
 2. The improved process according toclaim 1 further including withdrawing product gas from the other end ofsaid segregated storage adsorption zone.
 3. In a pressure swing processfor fractionating at least one component from a gaseous mixture byselective adsorption in each of at least two adsorption zones bysequentially passing the gaseous mixture from a feed stream through afirst adsorption zone until the zone is about saturated whilesimultaneously purging and then pressurizing a second adsorption zoneand then passing the gaseous mixture from the feed stream through thesecond adsorption zone until the zone is about saturated whilesimultaneously purging and then pressurizing the first adsorption zone,the improvement comprising withdrawing gas from one of said adsorptionzones at the end of the adsorption operation therein in a directioncountercurrent to feed flow and introducing the withdrawn gas along withthe gaseous mixture from the feed stream into the other of saidadsorption zones in a direction cocurrent with feed flow and afterpressurization thereof to equalize the pressures in said adsorptionzones from the feed ends thereof.
 4. A pressure swing process forfractionating at least one component from a gaseous mixture by selectiveadsorption in each of at least two adsorption zones comprising the stepsof:(a) providing a first adsorption bed having a gas inlet and a gasoutlet, at least one additional adsorption bed having a gas inlet and agas outlet, the gas inlets of said first and additional adsorption bedsbeing selectively connected to a feed gas stream, and a segregatedstorage adsorption bed isolated from direct communication with the feedgas stream; (b) withdrawing gas from said first adsorption bed at theend of the adsorption operation therein in a direction countercurrent tofeed flow and introducing the withdrawn gas along with the gaseousmixture from the feed gas stream into said additional adsorption bed ina direction cocurrent with feed flow and after pressurization thereof toequalize the pressures in said adsorption beds from the feed endsthereof; (c) terminating said pressure equalization of said beds fromthe feed ends thereof and simultaneously adsorbing the gaseous mixturefrom the feed gas stream in said additional adsorption bed, releasingproduct gas from the outlet of said additional adsorption bed andequalizing the pressures of said first adsorption bed and saidsegregated storage adsorption bed by withdrawing low purity gasincluding the mass transfer zone in said first bed from said firstadsorption bed in a direction cocurrent with feed flow and introducingsaid low purity gas and said mass transfer zone into one end of saidsegregated storage adsorption bed; (d) terminating said pressureequalization of said first and segregated storage adsorption beds andsimultaneously adsorbing the gaseous mixture from the feed gas stream insaid additional adsorption bed, and depressurizing said first adsorptionbed in a direction countercurrent to feed flow and purging said firstadsorption bed by diverting product gas from the outlet of saidadditional adsorption bed into said first adsorption bed in a directioncountercurrent to feed flow; (e) terminating said depressurizing andpurging of said first adsorption bed and simultaneously adsorbing thegaseous mixture from the feed gas stream in said additional adsorptionbed, releasing product gas from the outlet of said additional adsorptionbed and equalizing the pressures of said segregated storage adsorptionbed and said first adsorption bed by withdrawing gas from said one endof said segregated storage adsorption bed and introducing the withdrawngas and said mass transfer zone into said first adsorption bed in adirection countercurrent to feed flow; and (f) thereafter consecutivelyrepeating said steps beginning with pressure equalization of said bedsfrom the feed ends thereof reversing the functions of said firstadsorption bed and said additional adsorption bed.
 5. The processaccording to claim 4, wherein the gaseous mixture is air and the productgas is high purity oxygen.
 6. The process according to claim 4, furtherincluding controlling said process steps in a manner such that saidprocess is continued for a predetermined number of cycles.
 7. Theprocess according to claim 4, further including controlling said processsteps in a manner such that said process is terminated after twocomplete cycles.
 8. In a system for fractionating at least one componentfrom a gaseous mixture by pressure swing adsorption including a firstadsorption bed having a gas inlet and a gas outlet at least oneadditional adsorption bed having a gas inlet and a gas outlet, means forconnecting said gas inlets of said adsorption beds to a feed gas stream,a product outlet, and means for coupling said gas outlets of saidadsorption beds to said product outlet, the improvement comprising:(a)means defining a gas flow path connected at one end to said gas outletof said first adsorption bed and at the opposite end to said gas outletof said additional adsorption bed; (b) a first flow control valve insaid gas flow path between said gas outlet of said first adsorption bedand said additional adsorption bed, said valve automatically allowingunrestricted gas flow in a direction from said outlet of said first bedtoward said additional adsorption bed and controlled flow in a directionto said gas outlet of said first bed; and (c) a second flow controlvalve in said gas flow path between said gas outlet of said additionaladsorption bed and said first adsorption bed, said valve automaticallyallowing unrestricted gas flow in a direction from said outlet of saidadditional bed toward said first adsorption bed and controlled flow in adirection to said gas outlet of said additional bed.
 9. The improvedsystem according to claim 8 further including isolation valve means insaid gas flow path between said gas outlets of said adsorption beds. 10.The improved system according to claim 8 wherein said product outletcoupling means is connected to said gas flow path between said first andsecond flow control valves.
 11. In a system for fractionating at leastone component from a gaseous mixture by pressure swing adsorptionincluding a first adsorption bed having a gas inlet and a gas outlet, atleast one additional adsorption bed having a gas inlet and a gas outlet,means for connecting said gas inlets of said adsorption beds to a feedgas stream, a product outlet and output conduit means for coupling saidgas outlets of said adsorption beds to said product outlet, theimprovement comprising:(a) a reservoir; (b) first reservoir conduitmeans connected to said output conduit means and to said reservoir, saidfirst conduit means including flow control means allowing gas flow onlyin one direction from said output conduit means to said reservoir; and(c) second reservoir conduit means connected to said reservoir and tosaid output conduit means, said second conduit means including valvemeans for totally controlling flow of product gas from said reservoir tosaid output conduit.
 12. The improved system according to claim 11further including valve means in said output conduit between said secondreservoir conduit and said adsorption beds and control means operativelyconnected to said reservoir conduit valve means and to said outputconduit valve means for opening said reservoir valve means in responseto closure of said output valve means for supplying product gas fromsaid reservoir to said product outlet.
 13. The improved system accordingto claim 11 further including third reservoir conduit means connected atone end to said reservoir and coupled at the other end to saidadsorption beds, said third conduit means including flow control meansfor controlling flow of product gas from said reservoir to saidadsorption beds for operations such as purging and repressurization. 14.In a system for fractionating at least one component from a gaseousmixture by pressure swing adsorption including a first adsorption bedhaving a gas inlet and a gas outlet, at least one additional adsorptionbed having a gas inlet and a gas outlet, means for connecting said gasinlets of said adsorption beds to a feed gas stream, a product outlet,means for coupling said gas outlets of said adsorption beds to saidproduct outlet, and system control means for controlling systemoperation including sequentially passing the gaseous mixture from a feedstream through said first adsorption bed until said bed is aboutsaturated while simultaneously purging and then pressurizing saidadditional adsorption bed and then passing the gaseous mixture from thefeed stream through said additional adsorption bed until said bed isabout saturated while simultaneously purging and then pressurizing saidfirst adsorption bed, the improvement comprising: additional controlmeans operatively connected to said system control means for causingsaid system control means to maintain operation of said system for apredetermined number of cycles.
 15. The improved system according toclaim 14 wherein said additional control means causes said systemcontrol means to maintain operation of said system for at least twocomplete cycles.
 16. The improved system according to claim 14 whereinsaid additional control means causes said system control means toterminate operation of said system only after at least two completecycles.
 17. The improved system according to claim 14 wherein saidadditional control means causes said system control means to terminateoperation of said system only when the pressures in said adsorption bedsare substantially equal.
 18. The improved system according to claim 14further including gas product storage means connected to said productoutlet and pressure sensing means operatively associated with saidstorage means, said system control means being responsive to gaspressure in said storage means signalled by said sensing means forstopping operation of said system normally when gas pressure in saidstorage means reaches a predetermined magnitude, said additional controlmeans being operatively connected to said pressure sensing means andconnected in controlling relation to said system control means foroverriding said system control means to terminate operation of saidsystem only at a predetermined time.
 19. A pressure swing adsorptionsystem for fractionating at least one component from a gaseous mixtureby selective adsorption in at least two adsorption zones comprising:(a)a first adsorption bed having a gas inlet and a gas outlet and at leastone additional adsorption bed having a gas inlet and a gas outlet; (b)means for connecting said gas inlets of said adsorption beds selectivelyto a feed gas stream; (c) means defining a gas flow path connected atopposite ends to said gas outlets of said adsorption beds, a first flowcontrol valve in said path adjacent said outlet of said first adsorptionbed and automatically allowing unrestricted flow from said bed throughsaid valve and controlled flow through said valve toward said bed, and asecond flow control valve in said path adjacent said outlet of saidadditional adsorption bed and automatically allowing unrestricted flowfrom said additional bed through said valve and controlled flow throughsaid valve toward said bed; (d) output conduit means having a gasproduct outlet at one end and coupled at the other end to said gasoutlets of said adsorption beds; and (e) a reservoir, first conduitmeans connected to said output conduit means and including flow controlmeans allowing gas flow only in a direction from said output conduitmeans into said reservoir, and second conduit means connected to saidreservoir and to said output conduit means adjacent said gas productoutlet and including flow control means for controlling gas flow fromsaid reservoir to said gas product outlet through said output conduitmeans.